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Module 104: Nutrition, Metabolism and Excretion > Theme 1: Lecture 4 - Mobility of the GI tract > Flashcards

Theme 1: Lecture 4 - Mobility of the GI tract Flashcards

1
Q

What is the role of the GI tract

A

to extract chemical energy, vitamins, minerals and water from ingested products

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2
Q

What are the basic 4 layers of the GI tract

A
  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa
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3
Q

What is in the mucosa layer of the GI tract

A
  • Epithelium
  • Lamina propria
  • Muscularis mucosae
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4
Q

What is in the submucosal layer of the GI tract

A
  • Submucosal nerve plexus

- Blood vessels and lymphatic supply

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5
Q

What is in the muscularis externa layer of the GI tract

A
  • Circular muscle
  • Myenteric nerve plexus
  • Longitudinal muscle
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6
Q

Describe how the motility of the GI tract is controlled

A
  • Motility is governed by involuntary contraction of smooth muscle with pacemaker interstitial cells of Cajal (ICC)
  • Except upper oesophagus and external anal sphincter (striated skeletal muscle/voluntary)
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7
Q

Describe the smooth muscle in the GI tract

A
  • Smooth muscle is single unit- gap junctions allow electrical coupling and contraction as a functional syncytium
  • Smooth muscle organised into connected bundles of outer longitudinal and inner circular smooth muscle in muscularis layer
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8
Q

What is the intrinsic enteric nervous system

A
  • It controls GI motility and secretion independently of external neurostimulation
  • Reflex contraction in response to local stimuli (stretch, nutrients, irritation, hormones)
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9
Q

What are the 2 interconnected plexuses in the gut wall

A
  • Myenteric plexus

- Submucosal plexus

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10
Q

What is the extrinsic nervous control of the GI tract

A
  • Autonomic sympathetic and parasympathetic innervation

- Allow central modification to the GI tract

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11
Q

Describe the myenteric plexus

A
  • AKA Auerbach’s plexus
  • In the muscularis layer
  • Controls motility
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12
Q

Describe the submucosal plexus

A
  • AKA Meissner’s plexus
  • In the submucosal layer
  • Controls secretion and local blood flow
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13
Q

Describe the parasympathetic innervation of the GI tract

A

excitatory to motility and secretion (via Vagus and pelvic splanchnic nerves)

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14
Q

Describe the sympathetic innervation of the GI tract

A

Inhibitory to motility and secretion (via thoraco-lumbar innervation)

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15
Q

Describe how endocrine hormones enter the portal blood circulation

A

Endocrine hormones are secreted by entero-endocrine cells in the epithelial layer of the GI mucosa and enter portal blood circulation

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16
Q

Name 2 hormones that affect GI motility

A
  • Cholecystokinin CCK

- Motilin

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17
Q

Describe cholecystokinin

A
  • The stimulus for secretion is fat, protein and acid
  • It is secreted from I cells of the small intestine
  • Stimulates pancreatic secretions, gallbladder contraction and growth of exocrine pancreas. Inhibits gastric emptying
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18
Q

Describe motilin

A
  • The stimulus for secretion is fat, acid and nervous
  • Secreted from the M cells of the duodenum and the jejunum
  • Stimulates gastric and small intestine motility
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19
Q

What are the 2 types of electrical activity in smooth muscle cells of the gut

A
  • Slow waves

- Spike potentials

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20
Q

Describe slow waves

A
  • cyclical oscillations of membrane potential spontaneously initiated by pacemaker ICCs
  • Don’t reach threshold potential so don’t cause contractions in themselves
  • Provide a basic electrical rhythm
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21
Q

Describe spike potentials

A
  • Generated once threshold is reached resulting in Ca2+ influx and smooth muscle contraction
  • Causes contraction by further depolarisation to threshold levels
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22
Q

What is depolarisation of smooth muscle in the gut stimulated by

A
  • stretch
  • hormones (motilin)
  • excitatory neurotransmitter acetylcholine release from ENS excitatory motor neurons or P/S
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23
Q

What causes inhibition of the smooth muscle in the gut resulting in hyperpolarisation

A
  • inhibitory ENS
  • sympathetic NT norepinephrine
  • hormones (secretin)
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24
Q

What are the 2 types of contraction that occur in the gut

A
  • Segmentation

- Peristalsis

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25
Q

Describe segmentation contraction in the gut

A
  • For mixing
  • Bursts of circular muscle contraction and relaxation
  • Back and forth pendular movements also occur
  • stretch receptors trigger myenteric stimulation of muscle contraction
  • No net movement
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26
Q

Describe peristalsis contraction in the gut

A
  • For propulsion
  • local distension triggers contraction behind bolus and relaxation in-front
  • Wave of contraction
  • Requires functional myenteric plexus
  • Law of the intestines: movement aborally (away from mouth)
27
Q

Name a disease resulting from ENS innervation dysfunction

A

Hirschsprung’s disease

28
Q

What is Hirschsprung’s disease

A
  • A rare congenital absence of the myenteric plexus, usually involving a portion of the distal colon
  • The pathologic aganglionic section of colon lacks peristalsis and undergoes continuous spasm, leading to functional obstruction and severe constipation
29
Q

What type of epithelium is the oesophagus lined by

A

Stratified squamous epithelium

30
Q

What are the 3 stages of swallowing (deglutition)

A
  • Oral - voluntary initiation of swallowing in the oral cavity
  • Pharyngeal – involuntary passage of food through pharynx into oesophagus
  • Oesophageal – involuntary passage of food from pharynx to stomach
31
Q

Describe the oral phase of swallowing

A
  • Under voluntary control
  • tongue pushes up against hard palate and contracts to force lubricated bolus into the pharynx
  • Bolus enters the oropharynx initiating the pharyngeal stage through stimulation of sensory receptors
32
Q

Describe the oesophageal phase of swallowing

A
  • Motor efferents in trigeminal, glossopharyngeal and vagal nerves cause series of muscle contractions moving bolus through oropharynx into laryngopharynx and into oesophagus (nasopharynx is not involved in swallowing)
  • Soft palette elevates over posterior nares to close nasal pharynx
  • Epiglottis closes larynx
  • Respiration is inhibited
  • Upper oesophageal sphincter relaxes
  • Pharyngeal muscle contraction propels bolus into oesophagus
33
Q

Where is the swallowing centre

A

In the medulla oblongata and the pons in the brain stem

34
Q

Describe the oesophageal phase of swallowing

A
  • Primary peristalsis moves bolus downwards
  • Circular muscle contracts behind bolus, longitudinal muscle contracts in front to shorten fibres and push wall outward
  • Mucus lubricates and reduces friction
  • Relaxation of the lower oesophagus and lower oesophageal sphincter (LOS) occurs
  • Secondary peristalsis stimulated by stretch
  • Coordination is via intrinsic myenteric and extrinsic vagal innervation
35
Q

Name 2 diseases caused by oesophageal motility dysfunction

A
  • Achalasia

- Gastro-oesophageal reflux

36
Q

What is achalasia

A
  • LOS (lower oesophageal sphincter) fails to relax causing food to remain in oesophagus
  • Cause may be vagal or myenteric defect
  • Distention, inflammation, Infection and ulceration
37
Q

What is gastro-oesophageal reflux

A
  • LOS tone lost leading to flow of acidic gastric contents into oesophagus
  • Inflammation, ulceration
  • May be linked to hiatus hernia where portion of stomach protrudes through diaphragm into thorax causing gastric reflux
38
Q

What are the 3 primary motor functions of the stomach

A
  • Storage
  • Mixing
  • Emptying contents into the duodenum at a controlled rate
39
Q

Rugae folds

A

coiled sections of tissue that exist in the mucosal and submucosal layers of the stomach. They provide elasticity by allowing the stomach to expand when a bolus enters it

40
Q

Describe the storage function of the stomach

A
  • The vagovagal reflex mediates receptive relaxation reducing muscle tone and allowing reservoir function
  • The fundus (hollow part of the organ) functions primarily as a reservoir for storage of stomach contents
41
Q

Describe mixing in the stomach

A
  • Slow peristaltic waves are initiated in the body of the stomach moving stomach contents towards pyloric antrum.
  • Food is forced back for further mixing and digestion. This process of propulsion and retropulsion occurs in cycles to produce chyme
42
Q

Describe emptying of the stomach contents

A
  • Highly regulated with primary inhibitory feedback signals from small intestine
  • More powerful peristaltic contractions build to force chyme into the duodenum
43
Q

What is the excitatory regulation for emptying of the stomach

A

ENS/ANS neuronal stimulation and hormones eg motilin

44
Q

What is the inhibitory regulation for emptying of the stomach

A

ANS regulation, duodenal enterogastric reflexes and hormones eg CCK, secretin

45
Q

Name 2 diseases causes by gastric motility dysfunction

A
  • Dumping syndrome

- Gastroparesis

46
Q

What is dumping syndrome

A
  • Rapid emptying of gastric contents into the small intestine
  • Occurs following ingestion of large meal after gastrectomy
  • characterized by nausea, pallor, sweating, cramps, vertigo, and sometimes fainting within minutes
  • May be caused by hypertonic duodenal contents causing rapid entrance of fluid
47
Q

What is gastroparesis

A
  • Stomach fails to empty
  • prevents proper digestion
  • Causes bloating and nausea
  • May be caused by gastric cancer or peptic ulcers
  • occasionally observed through impaired vagal stimulation to the stomach in severely diabetic patients who develop autonomic neuropathy
48
Q

Describe how the small intestine has a large surface area

A

-circular folds (plicae circulares)
villi projections of the mucosa
-‘brush border’ microvilli on the epithelial cell apical surface

49
Q

Describe the 2 types of mixing in the small intestine

A
  • Mixing and circulation for maximum exposure to absorptive epithelium
  • Propulsion of chyme aborally
50
Q

Which hormones stimulate propulsive peristalsis in the small intestine (5)

A

gastrin, CCK, insulin, motilin, serotonin

51
Q

Which hormones inhibit propulsive peristalsis in the small intestine

A

secretin and glucagon

52
Q

Gastroenteric reflex

A

gastric distention activates myenteric plexus to promote SI (small intestine) peristalsis

53
Q

Gastroileal relfex

A

gastric distention promotes peristalsis in the ileum to force chyme through ileocecal valve into caecum

54
Q

Migrating motor complex (MMC)

A
  • Series of peristaltic contractions, between meals, every 90 mins sweeps contents into colon
  • Intrinsic enteric control, hormone motilin
  • Absence can lead to bacterial overgrowth
55
Q

Ileocecal valve

A
  • Controls emptying of chyme into colon

- Prevents backflow

56
Q

Name 3 types of disruption to peristalsis

A
  • Peristaltic rush
  • Paralytic ileus
  • Vomiting
57
Q

Peristaltic rush

A

mucosal irritation, ENS and ANS neural reflexes rapidly sweep contents of SI into colon

58
Q

Paralytic ileus

A

loss of peristalsis following mechanical trauma

59
Q

Vomiting (disruption to peristalsis)

A

reverse peristalsis initiated in distal small intestine to expel intestinal and gastric contents

60
Q

Why is motility in the large intestine more sluggish

A

To allow optimal:

  • Absorption of water and electrolytes (proximal)
  • Formation and storage of faeces (distal)
  • Commensal microbiome aids digestion, synthesises B and K vitamins
61
Q

Describe the longitudinal muscle in the large intestine

A

Longitudinal muscle in muscularis thickened to form three bands, taniae coli, which tonically contract to form haustral bulges

62
Q

Describe motility in the large intestine

A

-mixing contractions via haustral churning

Peristalsis:

  • mass movements occur 2-3x per day (forceful peristaltic contractions force contents into sigmoid colon and rectum)
  • gastro-colic and duodeno-colic reflexes: mass movements occur after meals on stretching via ANS
63
Q

Defecation reflex

A
  • Initiates defecation to expel faeces containing residues of digestion, bacteria, bile pigment, mucosal debris
  • mass movements push faecal matter into the normally empty rectum
  • stretch receptors are stimulated and activate the ENS and parasympathetic ANS
  • Involuntary contraction of longitudinal muscle in the rectum opens the internal anal sphincter
  • The constricted external anal sphincter is voluntarily relaxed to allow defecation

Module 104: Nutrition, Metabolism and Excretion (23 decks)

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